Vehicle battery system

A vehicle battery system includes: a first storage battery supplying electric power to a starter; a second storage battery connected in parallel with the first storage battery and including an internal switching unit switching a connection state with the first storage battery; an alternator connected to the first and second storage batteries and driven by an engine so as to generate electric power; a charging state detection unit detecting a charging state of the second storage battery: a charging control unit controlling a power generating state of the alternator and a switching state of the internal switching unit based on a detection result of the charging state detection unit; and a power generation margin determination unit determining whether there is a margin in the power generation of the alternator or not. When the charging state detection unit detects that a charging rate of the second storage battery is not higher than a minimum threshold, and the power generation margin determination unit determines that there is no margin in the power generation, the charging control unit controls the internal switching unit into an open state.